The present invention relates generally to material handling devices, and more particularly provides an improved forklift device which may be removably secured to the rear end of a flat bed truck for transport therewith, and then removed from the truck and used to unload articles carried on its bed portion.
Forklift devices of this general type are known in the material handling art, and one such device is illustrated and described in U.S. Pat. No. 4,395,190 to Barchard, while other related forklift apparatus is shown in U.S. Pat. No. 3,826,393 to Carroll, U.S. Pat. No. 3,908,849 to Carroll, U.S. Pat. No. 4,402,644 to Barchard, and U.S. Pat. No. 4,392,541 to Barchard.
The forklift illustrated in U.S. Pat. No. 4,395,190 to Barchard has an articulated frame in which a pair of front drive wheels are pivotally connected to the balance of the frame so that the wheels can be raised or lowered to horizontally lengthen or shorten the frame. Carried on a horizontally central portion of the frame is a vertically extending mast structure which supports a pair of forwardly extending lifting forks that may be moved upwardly or downwardly along the mast structure. The forks are telescoped within a pair of sleeve elements which may be hydraulically extended or retracted relative to the forks to increase or decrease the effective horizontal reach thereof.
To unload a palletized article resting on the bed of a flat bed truck adjacent its "near side" edge, a relatively simple and straightforward unloading technique is used in which the Barchard forklift is driven toward the near side truck bed edge, with the fork extensions positioned at the pallet level, until the fork extensions enter the pallet openings. The fork extensions are then moved upwardly against the mast structure to lift the article upwardly from the truck bed. Finally, the forklift is backed away from the truck and driven to an unloading location, and the article is lowered to the ground and removed from the forks.
The unloading of "far side" articles from adjacent the near side of the truck bed, however, is considerably more complex and difficult. To unload a far side article, the forks are elevated to an appropriate level and the front drive wheels are downwardly pivoted so that the mast structure is moved upwardly and brought horizontally closer to the front wheels of the fork lift so that the mast structure can be brought closer to the near side of the truck bed. The fork extensions are then moved horizontally across the upper surface of the truck and inserted into the pallet of the far side article to be unloaded. The forks are then lifted to lift the article a short distance above the truck bed and the fork extensions are then retracted to move the supported article toward the mast structure. When the fork extensions have been retracted, the fork lift is then backed away from the truck as previously described for near side article unloading.
One of the problems often encountered in this far side unloading process is that the far side article, when initially lifted, tends to tip the fork lift forwardly due to the forward shift of its center of gravity caused by downwardly pivoting the drive wheels, and the rather large forward extension of the fork structure. Because of this tendency to forwardly tip, the far side article often cannot be completely lifted off the truck bed and then carried rearwardly by retracting the fork extensions. Accordingly, it is often necessary to use the fork extensions to exert only a partial lifting load on the far side article and then attempt to drag the article across the upper truck bed surface toward the near side edge of the bed by retracting the fork extensions. This rather delicate operation can create a variety of undesirable results such as damaging the upper surface of the truck bed, pulling the fork extensions out of the pallet, or repeatedly tipping the forklift about its front wheels.
Additionally, even if the far side article is successfully moved to adjacent the near side edge of the truck bed, the previous forward movement of the mast structure relative to the forklift front drive wheels creates a concomitant forward shift in the center of gravity of the forklift so that when the article is lifted from the truck bed the resulting tipping load moment created by the article is resisted by a substantially reduced opposite moment created by the weight of the forklift itself. Accordingly, the forklift stability after the article has been lifted off the truck bed is substantially diminished.
Thus, while the Barchard forklift is capable of reaching across a truck bed to engage and rearwardly move an article, it is far easier and more desirable simply to drive the forklift around to the far side of the truck bed so that the far side article becomes a greatly more accessible and maneuverable "near side" article. This, of course, adds significantly to the unloading time required to remove all of the articles from the truck bed.
It is accordingly an object of the present invention to provide improved forklift apparatus which can more safely, conveniently and quickly unload both near side and far side articles from a truck bed from the same side of the truck to thereby significantly reduce the total truck unloading time.